A comprehensive overview of carbon dioxide, including emission sources, capture technologies, and the conversion into value-added products

Carbon dioxide is both the planet’s enemy and friend since it maintains the balance of the planet as this gas retains some of the radiant energy that the planet receives. Thus, it keeps the Earth’s heat, without which the Earth would be unbearably cold. However, the increase in the quantity of CO2 in the Earth’s atmosphere contributes to the exacerbation of global warming. CO2 is mainly produced from the processes of combustion of fuel and petroleum, as well as from the breathing process of living organisms. This study aims to provide a comprehensive view of carbon dioxide, including the main sources of emissions that resulted in negative effects, as well as the various ways to control these emissions by conducting a separation process during fuel combustion in power plants and preventing the release of large quantities into the atmosphere. Finally, an objective viewpoint on how to benefit from separated carbon dioxide and turn it into highly valuable products is presented. Given that this is a global problem that affects our planet, this review links other literature that deals with each part separately, and thus it presents a clear and comprehensive vision that enables the reader to be aware of carbon dioxide gas from all perspectives, concluding with future studies that are required to prevent an increase in negative effects in order to preserve the quality of life in our planet.


Introduction
Fossil fuel combustion by-products such as toxic air pollutants like polycyclic aromatic hydrocarbons, nitrogen dioxide, atmospheric ammonia, and the major climatealtering gas carbon dioxide represent a serious risk to health (Perera et al. 2019;Perera 2018;Zhang et al. 2020) and result in global warming (Inumaru et al. 2021;Janke et al. 2014b).By the end of the century, continuing higher emissions of greenhouse gases could increase greenhouse gas concentrations in the atmosphere to approximately four times pre-industrial levels.This would be expected to bring a 3.7 °C rise in global mean surface temperatures (with a likely range of 2.6 °C to 4.8 °C) by the 2090s compared to 1986-2005 temperatures, in addition to the warming of 0.6 °C which has already occurred since preindustrial times.Such warming would cause severe disruptions to precipitation patterns, as well as to the frequency and intensity of some extreme weather events such as heat waves, and storm surges in coastal areas (Change 2013;Organization 2015).A 2 °C upper limit of temperature increase relative to pre-industrial levels has been internationally agreed in order to prevent severe and potentially catastrophic impacts from climate change.Reductions are necessary across countries and sectors.In order to stay below the 2 °C upper limit, it is estimated that global annual CO 2 emissions, which are currently at 5.2 tons per capita, need to be reduced to 1.6 tons per capita (Organization 2015).
Carbon dioxide is considered one of the primary causes of acid rain, along with sulfur dioxide and nitrogen oxides, as CO 2 interacts with water in the atmosphere, forming acid rain.Although the interaction of carbon dioxide with water produces a weak acid (carbonic acid), unlike sulfur and nitrogen oxides that produce strong acids (sulfuric and nitric acids) (Eldesouki et al. 2022;El-Khatib et al. 2004;El-Moneim et al. 2011), the increase in carbon dioxide emissions will exacerbate the situation (Singh and Agrawal 2007).Marine biologists have expressed grave alarm about the potential effects of ocean acidification on marine life.Alteration of one species or a group of species due to acidification will certainly negatively affect the ecosystem, especially food.Because calcification is anticipated to be hampered by increased seawater acidity, Marine calcifiers are thought to be particularly vulnerable.Additionally, acidified saltwater is regarded as "corrosive" and can lead to the dissolution of CaCO 3 minerals and the fragilization of calcareous structures.Furthermore, acidosis brought on by ocean acidification has the potential to impair numerous crucial physiological functions, including aerobic metabolism.In a result, metabolic depression slows down the energy production needed to maintain calcification as well as several other biological processes and activities (Fabry et al. 2008;Pörtner 2008;Leung et al. 2022).The effects of acid deposition may currently be almost imperceptible, since 1850, the pH of seawater has already reduced by 0.1, and by 2100, it is expected to decrease by 0.3 (Figuerola et al. 2021), but with the increase in acidity, more and more species of plants and animals may decrease or even disappear.As the water's pH approaches 6.0, some crustaceans, insects, and plankton begin to disappear.As the pH approaches 5.0, significant changes occur for some fish group (Mehta 2010).
High CO 2 emission has both direct and indirect effects on human health.As carbon emissions are the main factor of climate change.Extreme heat waves, sea level rise, changes in precipitation that lead to floods and droughts, and powerful hurricanes can lead to some health problems and even death (Masson-Delmotte et al. 2021b;Ebi et al. 2018).For instance, worsening air pollution levels may have detrimental effects on cardiovascular and respiratory disorders (Mills et al. 2009).High carbon emissions also have a substantial negative impact on mental health and well-being (Ebi et al. 2021).According to certain studies (Ngarakana-Gwasira et al. 2016;Parham and Michael 2010), endemic malaria will become a bigger issue in the African highlands as a result of climate change.Temperature variations have an impact on the transmission of malaria by slowing or accelerating it.The amount of rainfall also has an impact on malaria transmission as it creates a lot of breeding grounds for mosquitoes (Mafwele and Lee 2022).
As we have previously stated, the results of increased carbon dioxide emissions will be disastrous for our planet, and therefore it is important to maintain acceptable levels of CO 2 in the atmosphere and not completely get rid of it because it has many benefits in preserving the normal temperature of our planet.On November 20, 2022, the twenty-seventh Conference of the Parties to the United Nations Framework Convention on Climate Change (COP27), which was held in the Egyptian coastal city of Sharm el-Sheikh, concluded with some important key results that lead to a reduction in harmful carbon emissions, and the most important results are reducing the temperature to about 1.5 °C by encouraging strategies that lead to this, providing financing for losses and damages to vulnerable countries severely affected by floods, droughts and other climate disasters, a new focus on accountability when it comes to commitments made by sectors, companies and institutions as well as strengthening the actions taken by countries to reduce emissions greenhouse gases and adaptation to the inevitable effects of climate change, along with strengthening financing, technology and capacity building support needed by developing countries (Alayza et al. 2022).
The novelty of the review lies in presenting an integrated picture of carbon dioxide, including the various aspects that surround the gas from the main sources that cause its emissions, such as fuel combustion, and some statistics related to carbon emissions such as most CO 2 -emitting countries in the previous years.The research also deals with the most important strategies for controlling carbon emissions, like capturing and storing carbon dioxide using various technologies as well as converting it into synthetic fuels and products of economic value.This gives the reader a comprehensive view of carbon dioxide and thus leads to gaining more support and interest in this topic.

CO 2 emissions sources
Carbon dioxide emissions come from a variety of sources, including both natural and industrial sources.Decomposition, ocean release, and respiration are examples of natural sources.Human-caused sources include steel and cement industries, deforestation, and the consumption of fossil fuels such as coal, oil, and natural gas (Yin et al. 2020;Sher et al. 2021a, b).Globally, total carbon dioxide emissions from fossil fuel combustion and industrial activities in 2000 amounted to about 24.6 gigatons (Gt) CO 2 compared to 36.5 in 2021, an increase of about 48.4% (Metz et al. 2005;Atlas 2019;IEA 2021).By comparing the percentage of emissions resulting from the combustion of fossil fuels with those resulting from various industrial processes and biomass in 2020 as shown in Fig. 1 a, we find that CO 2 emissions from fossil fuels amounted to about 78%, while emissions from biomass were less than 1% (Ritchie et al. 2020).What makes carbon dioxide the main cause of global warming is that it is the main component of greenhouse gases, as Fig. 1 b shows that carbon dioxide has the largest proportion of greenhouse gases (79%), followed by methane (11%) and nitrous oxide (7%) (Hockstad and Hanel 2018).By comparing the emissions from the various energy sectors in the years 2000 and 2021 as shown in Fig. 1c, we find that the emissions resulting from coal are the largest compared to other sectors  (Metz et al. 2005;IEA 2021) of natural gas, crude oil and biomass in the two years, as the emissions resulting from coal doubled during the past two decades to reach 15 Gt CO 2 .In 2021, emissions from coal were about twice as high as those from natural gas, and 30% more than those from crude oil (Metz et al. 2005;IEA 2021).The sectors of transportation, electricity generation, and industry are among the sectors that consume the most fuel and therefore emit the most carbon dioxide, especially in industrialized countries such as the United States of America, where carbon emissions from these sectors in 2020 amounted to about 80% of the total emissions as shown in Fig. 2, transportation represents about 33%, followed by electricity generation by 31% (Solaymani 2019;Hockstad and Hanel 2018).
The major industrialized countries are the largest emitters of CO 2 .Figure 3 shows the top ten countries that emitted the most carbon dioxide from 1990 to 2019.In 2019, China, whose emissions exceeded 10 Gt CO 2 , was at the top of the list, and the United States of America came second with nearly half of this amount, then India with around 2.5 Gt CO 2 , while Iran, South Korea, Indonesia and Canada, respectively, occupied the final four positions on the list, with a quantity that exceeded 0.5 Gt CO 2 (Agency 2021;Hockstad and Hanel 2018;Ritchie et al. 2020).Referring to the beginning of this period, the United States of America was in first place with emissions estimated at about 5.0 Gt CO 2 , while China came second with emissions of about 2.4 Gt CO 2 (Atlas 2019).Table 1 represents the amount of emissions per capita in the countries that emit the most carbon dioxide during the past thirty years (Crippa et al. 2022), but it should be taken into account that the population varies from one country to another, and the population in the same country also varies over the years.For example, America ranks third in the world in terms of population, as it reached about 309 million in 2010.While the year 1990 was about 252 million (Crippa et al. 2020;Desa 2019).That is, the increase is about 57 million.In comparison, the amount of CO 2 emitted in 2010 amounted to 5.5 Gt CO 2 and 5.0 Gt CO 2 in 1990.Therefore, although the amount of emissions per capita decreased by about 2%, the actual emitted quantities increased by approximately 0.5 Gt CO 2 .
Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon cycle (the natural circulation of carbon between the atmosphere, oceans, soils, plants and animals).Human activities are altering the carbon cycle-by adding more carbon dioxide to the atmosphere Fig. 2 CO 2 emission sources in the USA in 2020 (Solaymani 2019;Hockstad and Hanel 2018) Fig. 3 The top ten CO 2 emitter countries from 1990 to 2019 (Agency 2021;Hockstad and Hanel 2018;Ritchie et al. 2020) and affecting the ability of natural sinks, such as forests and soils, to remove and store carbon dioxide from the atmosphere.While CO 2 emissions come from a variety of natural sources, emissions associated with humans are responsible for the increase in the atmosphere since the industrial revolution that has led to global warming (Masson-Delmotte et al. 2021a).However, future projections indicate that by 2050, emissions will be reduced to about 50% of global carbon dioxide (Priddle 2002).This may relate to global awareness of the risks posed by high CO 2 emissions and the world trend to replace fossil fuels with clean energy such as hydrogen and methanol fuel.
It is important to implement certain economic policies, especially with regard to primary sources of carbon dioxide emissions, in order to reduce energy consumption from fossil fuel combustion.Some of these policies include discouraging the use of certain fuels and technologies, such as coal-fired power plants, gas boilers, and conventional internal combustion engine automobiles.Massive investment in infrastructure, including smart transmission and distribution networks, must also be planned for and encouraged by governments.Heavy industrial facilities must be equipped with Carbon Capture, Utilization and Storage techniques (CCUS) by 2030, new hydrogen-based plants must be constructed, and regulations banning the sale of new cars with internal combustion engines and replacing them with electric ones will lead to a significant reduction in transportation emissions.By 2050, all cars on the road must be powered by electricity or fuel cells.Increased reliance on low-emission fuels, for example, aviation should largely rely on biofuels and synthetic fuels.In terms of buildings, most old buildings and all new buildings must be modified to comply with clean energy uses as carbon-neutral.Globally, 1.6 Gt of CO 2 should be captured annually by 2030, rising to 7.6 Gt by 2050.Approximately 95% of all CO 2 captured will be kept in permanent geological storage, with the remaining 5% being used to generate synthetic fuels (Bouckaert et al. 2021;Sher et al. 2021a, b).

CO 2 capture and storage
Carbon capture, use and storage can provide a major contribution to addressing the emissions.The technology options included in the Long-Term Strategy 2050 (Gielen et al. 2019;Bustos-Salvagno;Commission 2019) and "Sustainable Carbon Cycle Communications" (Plan 2018) are carbon dioxide capture from fossil fuel combustion or from industrial processes.The captured carbon is then either stored underground or used for the production of synthetic materials (fuels, chemicals, building materials).Carbon dioxide is captured from high sources such as power plants, steel and cement manufacturing plants by one of the following ways; chemical absorption, physical adsorption, membrane separation, cryogenic distillation and biofixation.The highly concentrated carbon dioxide is then compressed and transported through pipelines or ships to storage reservoirs (Svensson et al. 2004;McCoy and Rubin 2008) such as geologic formations (Anderson and Newell 2004) and oceanic storage (Adams and Caldeira 2008).

CO 2 capture technologies
There are three technological ways to capture CO 2 from power plants: pre-combustion capture, post-combustion capture and oxy-fuel capture as presented in Fig. 4. The post-combustion capture is useful for separating CO 2 from exhaust gases created by burning fossil fuels.The exhaust gases which contain mixture of CO 2 at concentration 13-15% (Wang et al. 2017), nitrogen and some oxides (SO 2 , NO 2 and O 2 ) are firstly treated to remove particulate matter and the oxides of nitrogen and sulfur.Generally, they are in contact with a liquid solvent, such as amine solution.The amine selectively absorbs the CO 2 , capturing more than 85% of the CO 2 and enabling nitrogen and oxygen to be released into the atmosphere (Ishaq et al. 2021).A CO 2 -rich amine is regenerated by stripping the CO 2 out of the liquid with steam, allowing the lean amine to be recycled to the absorber while producing a concentrated CO 2 stream (Qureshi et al. 2021).Finally, CO 2 is compressed and cooled in liquid form (Basile et al. 2011).Compared to other fossil fuel power plants, natural gas combined cycle (NGCC) plants are regarded as being quite efficient in producing electricity (Rubin et al. 2005).As NGCC gives the same efficiency in power generation with carbon emissions less than half of those produced by coal-fired plants, but they still release a large amount of CO 2 into the atmosphere about 350 kg CO 2 /MWh (Diego et al. 2017).Decarbonizing NGCC power plants using post-combustion technology is relatively complex due to the high flue gas flow rate and low CO 2 intensity of ~ 3-4 vol% with a residual content of 20% O 2 and 77% N 2 .Parallel and hybrid selective exhaust gas recirculation were investigated by (Qureshi et al. 2021), in which CO 2 was selectively recycled and mixed into the ambient air for the compressor input feed, lowering the flue gas flow rate and increasing CO 2 concentration at the capture plant's inlet.
The performance results suggested that the CO 2 content of the hybrid configuration is increased to approximately 19 vol% and 13-14 vol% for the parallel configuration.
In the pre-combustion CO 2 capture, the hydrogen rich gas is used as a low carbon fuel in a combined cycle plant where syngas are produced by adding oxygen to the primary fuel, the process is called partial oxidation when the primary fuel is gaseous or liquid fuel and referred as gasification when it is applied to solid fuel.This partial oxidation is done using oxygen which separated from air.The syngas production is followed by the water-gas shift (WGS) reaction to convert CO to CO 2 and H 2 by the addition of steam.The concentration of CO 2 at the inlet separation stage can ranges from 15 to 60%.(Gazzani et al. 2013;Jansen et al. 2015).
Oxy-fuel combustion is a new method of using enriched oxygen for fuel combustion with recycled flue gases to increase the CO 2 concentration in exhaust gases by up to 60-70% (Jurado et al. 2015) (compared to the post-combustion mode by about 13-15%) making it possible to separate (1) or capture pure CO 2 from the flue gas by means of lowtemperature dehydration and desulfurization processes (Carpenter and Long 2017; Yang et al. 2008).

CO 2 separation technologies
In general, to economically separate CO 2 from power plants, a relatively pure high-pressure stream of CO 2 must be produced.The process of producing the carbon dioxide stream is referred to as separation process.CO 2 separation is a critical and important process for natural gas purification, flue gas recycling from thermal cracking, and greenhouse gas mitigation (Xu et al. 2021).

Absorption
Chemical absorption involves reaction between the absorbent substance (CO 2 ) and the solvents to form a rich liquid, then the rich liquid enters the absorption tower to heat and decompose CO 2 (Xie et al. 2020).Solvents that have proven efficiency in the process of carbon dioxide capture and storage belong to alkanolamine family such as monoethanolamine (MEA), diglycolamine (DGA), diethanolamine (DEA), di-isopropanolamine (DIPA), triethanolamine (TEA), N-methyldiethanolamine (MDEA), 2-amino-2-methyl-lpropanol (AMP), and 2-piperidineethanol (2-PE) (Benamor et al. 2012).Carbon dioxide is absorbed using amines to form a soluble carbonate salt.The absorber operates at a temperature below 60 °C and ambient pressure.This reaction is reversible and carbon dioxide can be released by heating the solution with the carbonate salt in a separate separation column.Decarbonization takes place at 120 °C and pressures range from 1.8 to 3 bar (Vega et al. 2018).Despite MEA's efficacy in capturing CO 2 from flue gases.However, there are certain drawbacks, including major corrosion issues, a rapid degradation rate, a poor capacity, and a high energy consumption (Yeh et al. 2005;Zhang and Guo 2013).As an alternative to amines, ammonia is the most advantageous solvent for reducing CO 2 emissions and capturing CO 2 from power plants (Zhang and Guo 2017).(Ishaq et al. 2021) conducted a simulated model evaluation of an NH 3 -based post-combustion CO 2 capture process for two process optimizations: a rich-solvent split process and split flow arrangement configurations.Compared to the reference, the NH 3 -based process has shown a maximum reduction of 15.8% in re-boiler duty and an overall energy saving of 21.5% and the split flow arrangement is the most suitable modification to reduce the specific reboiler duty with aqueous ammonia and save energy by 32.8%.

Adsorption
Various materials such as zeolite and activated carbon (Kumar et al. 2020), have been investigated as adsorbent materials for CO 2 separation and capture by adsorption process.Among these materials, activated carbon is the most common and most effective modified adsorbent due to its high surface area and large pore size.The adsorption method of CO 2 capture involves filling the packed column with an absorbent and a CO 2 carrying stream is passed through the column.The carbon dioxide is attracted toward the absorbent material and sticks to the surface of the absorbent material.After achieving the equilibrium, the desorption can be carried out to obtain pure CO 2 and renewable adsorbent by raising the temperature (Krishnaiah et al. 2014).

Membrane separation
In membrane separation processes, the membrane acts as a selective barrier allowing relatively free passage of one component while retaining another.Polymeric membranes are of particular economic importance due to the low cost of the material and ease of large-scale manufacture.In a typical membrane gas separation process, the partial pressure of the penetrating gases is represented as the driving force, as gas molecules are first adsorbed on the upper side of the membrane, then diffuse across the membrane and finally adsorb on the lower side of the membrane.The membrane separation process can be explained by two mechanisms, the solution-diffusion mechanism, and the facilitated transport mechanism.The mechanism of solution-diffusion depends (3) mainly on permeability and selectivity.Permeability (P A ) is a product of the solubility coefficient (S A ) and the diffusion coefficient (D A ), and is given by Eq. 5 (Ebnesajjad 2016;Wu 2019).
The solubility of gas molecules in a membrane is directly proportional to gas condensability (characterized by the critical temperature) and interactions of gas molecules with the membrane matrix.Gas diffusion through the membrane is controlled by void space between polymer chains.Since the critical temperature of CO 2 (31 °C), is greater than other light gases (N 2 (−147 °C), CH 4 (−82 °C), H 2 (−240 °C)).Therefore, the solubility of CO 2 gas is the highest leading to CO 2 being the much more permeable gas (Wang et al. 2016).In facilitated transport mechanism, the reactive carriers in the membrane react reversibly with small molecules and hence, facilitate the transporting process.The CO 2 hydration reaction with primary and secondary amine gives carbamate ion, however, with tertiary amine produces bicarbonate as the following reactions: The CO 2 separation performance of the membranes is affected by the CO 2 carrier reaction kinetics.If carriers have a very strong affinity for CO 2 , they will react almost irreversibly, and the CO 2 will not be able to be released.If the carriers react slowly with CO 2 , the membrane will easily become saturated with carbon dioxide and easy transfer will not occur.Ideally, efficient facilitated transport requires a balance of fast responses (Wang et al. 2016).

Cryogenic distillation
In the cryogenic distillation, the gases mixture is separated using high pressure and low temperature.The stream gas is first dehydrated and then cooled with refrigeration to a temperature ranges from −100 to −135 °C followed by separation of solid CO 2 from the lighter gases.By this process, methane, ethane, propane and hydrogen sulfide can also be separated (Stewart and Arnold 2011).One of the most important features of this process is the ability to separate a large amount of carbon dioxide gas, which can reach 90-95%, as the gas is compressed into a liquid.However, the main problem is the amount of energy consumed, which was estimated at about 600-650 kW to recover one ton of CO 2 in liquid form (Hanif et al. 2022). (5)

Biofixation
Microalgae are microscopic organisms found in sea and fresh water.They can be classified into eukaryotic microorganisms or prokaryotic cyanobacteria (blue-green algae), and more than 25,000 species have been isolated from these organisms.These microorganisms carry out photosynthesis, an important natural mechanism for reducing the concentration of carbon dioxide in the atmosphere (Vale et al. 2020).Microalgae are considered attractive bio-factories for carbon dioxide sequestration and production of value-added products such as biofuels and food (Ryan et al. 2009;Harun et al. 2010).The ability of CO 2 capture by microalgae depends on many parameters such as microalgae species, cultivation system, temperature, pH, turbidity and salinity.One of the most important advantages of using microalgae is the ease of the method, as well as energy savings over other methods (Onyeaka et al. 2021).The results of some studies showed that the Chlorella sp. has the best ability to absorb CO 2 with high efficiency (Iglina et al. 2022).The majority of the microalgae are photoautotrophic as they convert inorganic CO 2 into carbohydrates via photosynthesis process (Prasad et al. 2021).In the photoautotrophic process, the most common pathway for synthesizing organic compounds from carbon dioxide is the reductive pentose phosphate (Calvin) cycle shown in Fig. 5 in which the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) plays acritical role in CO 2 sequestration.The enzyme catalyzes the reaction of CO 2 with ribulose-1,5-bisphosphate (RuBP) to form 3-phosphoglycerate (PGA) which is converted into the carbohydrate glyceraldehyde-3-phosphate.Some of this product exits the pathway that will be used for the synthesis of more complex carbohydrates or other carbon compounds and the rest is converted back into RuBP (the substrate for the initial CO 2 fixation reaction (Senatore et al. 2020).
Projects are now being supported to develop CO 2 capture technologies in more than 30 countries.For instance, a total capability to capture around 15 Mt CO 2 annually by 2030 may arise from more than ten projects throughout Southeast Asia, most of which have been announced since January 2020.Regarding North America, the USA has over 80 target projects prepared for execution by 2030 and may show a roughly fivefold increase in its annual capacity of carbon dioxide capture, from more than 20 Mt CO 2 to more than 100 Mt CO 2 .There are at least four projects in the Middle East that are in various stages of development.The North Field East LNG project in Qatar will increase the country's CCUS capacity by 2050 from more than 2 Mt CO 2 to 5 Mt CO 2 per year (Budinis et al. 2022).

CO 2 conversion
Converting CO 2 into valuable products is a huge challenge as it aims to eliminate millions of metric tons of carbon dioxide, which are emitted annually from power plants and lead to air pollution.The world is currently following some strategies to preserve the environment from the emissions that posed a threat in the past decades.These strategies include the relying on clean and renewable energy as an alternative to fossil fuels, the main cause of carbon dioxide emissions.In another direction, the efforts of researchers and scientists to find new ways and catalysts to achieve the largest conversion rate of carbon dioxide gas and produce fuels as well as industrial materials such as polymers and plastics.The process of converting carbon dioxide is not free from some difficulties, as carbon dioxide consists of one carbon atom covalently bonded by two bonds with two oxygen atoms.The bonds between oxygen and carbon atoms are very strong, which makes the molecules more stable, and to conduct reactions on carbon dioxide, these stability must be weakened (Aresta et al. 2014).In order to achieve this goal, three main obstacles must be overcome: large energy inputs must come from low carbon sources to avoid more carbon dioxide emissions, active catalysts to reduce the activation energy of carbon dioxide conversion processes and achieve the most conversion rate, and finally, suitable reaction conditions represented by high temperatures and pressures to weaken the stability of carbon dioxide (Alper and Orhan 2017;Kamkeng et al. 2021).The following discusses the Fischer-Tropsch technology as one of the important methods that are used in the process of producing valuable materials from carbon dioxide, as well as several valuable products from CO 2 conversion process.

Fischer-Tropsch technology
It was developed in the mid-1920s in Germany to produce liquid fuels from coal.What gives this technology the advantage is that it can produce types of liquid fuels at competitive prices for petroleum today (Suppes and Storvick 2016).Fischer Tropsch is a gas-to-liquid polymerization technology to convert carbon monoxide and hydrogen into liquid hydrocarbon fuels.It is an exothermic process through the use of a metal catalyst that causes polymerization (Darmawan and Aziz 2022), according to reaction (9).
It is clear from the previous description that the process of Fischer-Tropsch depends mainly on carbon monoxide, so to use this method for CO 2 , carbon dioxide must first be converted to carbon monoxide (Garba et al. 2021;Rashed et al. 2021).This is possible through the production of Syngas (CO, H 2 ) using the reverse water gas shift (RWGS) process as described in reaction ( 10).This reaction is thermodynamically favored at elevated temperatures (Hannula et al. 2020).
Since the development of effective catalysts enhances the RWGS pathway, selectivity can be increased also by optimizing the operating conditions such as temperature, pressure, H 2 /CO 2 ratio, or amount of catalyst (Portillo et al. 2023).There are two proposed mechanisms for RWGS reaction: the associative and redox mechanisms.In a redox process, CO 2 is reduced on an electron donor's surface to give CO and the electron donor which could be a metal or an oxygen vacancy in a reducible oxide is oxidized.Water is then produced when H 2 reduce the active center again to its primary state.The associative mechanism involves the reaction between CO 2 activated species on the surface and H dissociated species to produce reaction intermediates, which then break down to form the reaction products (González-Castaño et al. 2021).Depending on the catalyst, temperature, and type of process employed, hydrocarbons ranging from methane to higher molecular paraffins and olefins can be obtained (Rashed et al. 2022).Also, small amounts of low molecular weight oxygenates are produced (such as alcohol and organic acids).Over the years, three types of Fischer-Tropsch reactors have been used as described in Table 2 and Fig. 6; fixed bed reactor, slurry reactor, and fluidized bed reactor.

Fixed bed reactor
This type of reactor is used in low and high temperature Fischer-Tropsch processes where it operates between 180 and 350 °C, at pressures ranging from 10 to 45 bar (Botes et al. 2011).The process is characterized by the reaction of different gases on the surface of the catalyst placed in a fixed position inside the reactor.The feed gas follows the path of  Gunjal and Ranade (2016) a heat carrier that allows for control of different temperature profiles around the fixed tubular catalysts as shown in Fig. 7 (Nelabhotla et al. 2021;Rashed et al. 2023).Fixed bed reactors are simple and flexible, however, there are some disadvantages such as high cost and to avoid a high-pressure drop in the reactor tubes, large catalyst particles are required that lead to catalytic activity reduction (difficulty diffusion of reactants in the catalyst particle core), in addition, low heat transfer from the catalyst bed, and temperature change in the tubes make it difficult to control the production process (Schubert et al. 2001;Worstell 2014).

Slurry reactor
In order to get around the problems with fixed bed reactors, slurry reactors were created.The slurry reactor is easier to design and much cheaper.Also, it is characterized by rapid heat removal, and because the slurry phase is well mixed, the temperature inside the reactor is more stable.This means that greater temperatures can be reached without the risk of catalyst sintering (Rauch et al. 2013;Nasser et al. 2018).
Additionally, raising the slurry concentration will result in more large bubbles being formed, which will boost the reactor's productivity (Inglezakis and Poulopoulos 2006).A low pressure drop, wide catalyst surface area (Reay et al. 2013), simple catalyst removal, and low catalyst consumption are further significant benefits (Wang and Economides 2009).
In slurry reactors, the mobile catalytic (solid) phase coexists with the gas and liquid phases.Depending on the procedure, the stages might play a variety of roles.The gas phase can be one of the reactants or products, or an inert substance used to stir a liquid and solid mixture.Similarly, the liquid phase can participate in the catalytic reaction or act as an inert medium for the contact of the gas phase and solid catalyst (Avci and Önsan 2018).

Fluidized bed reactor
In fluidized bed reactors, the catalyst is positioned on a distributor plate on which a fluidized gas is passed, carrying the particles in a fluid state.This eliminates some of the issues with fixed bed reactors.As a result, catalysts are more available because they are well mixed in fluid and have a larger surface area to react on (Rajmohan et al. 2020;Zhang et al. 2019).Fluidized bed reactors also employ a heat carrier made up of small inorganic particles, which are very efficient at providing high heat-transfer rates while maintaining uniform bed temperatures (Gunjal and Ranade 2016;Guda et al. 2015).

Valuable products
The products of the carbon dioxide conversion process attract the attention of researchers to try to find methods and catalysts that help in this process, as it is possible to convert CO 2 to a substance of great industrial importance from fuels and monomers to polymers and plastics (Hassan et al. 2020).To simplify the idea, we will mention some practical examples of CO 2 conversion products.

Oxygenates
One of the top five most traded chemicals in the world is the simple oxygenated hydrocarbon methanol, or CH 3 OH (Verhelst et al. 2019).It is an easily available biodegradable and widely used chemical, which can be used in production of various value-added chemicals (Shee et al. 2022).Methanol is used as a volatile organic solvent in factories.It is also the most commonly used fuel in direct methanol fuel cells (DMFCs) (Cheshideh et al. 2022).The process of producing methanol from CO 2 is an important process because it helps reduce carbon emissions.The main methanol production reactions are (Previtali et al. 2020): The main factors in this process are the catalytic activity of the catalysts and the reaction conditions of temperature and pressure.The catalytic activity is assessed by the selectivity of the catalyst to methanol and the rate of methanol production.Other gases are often produced from the conversion process, such as carbon monoxide and methane (Xiong (11) CO (g)+ 2H 2(g) ⇌ CH 3 OH (g) ΔH = −136.9kJ∕mol (12) CO 2(g)+ 3H 2(g) ⇌ CH 3 OH (g) + H 2 O ΔH = −97.36kJ∕mol et al. 2019).The higher the selectivity and rate of methanol production, the higher the efficiency of the catalyst as shown in Table 3. Methanol can often be used in blends or in pure form in internal combustion engines in the transportation sector.Currently, M85 (a mixture of 85 vol% methanol and 15 vol% gasoline) or M100 (pure methanol) can be used in the spark-ignited combustion engines of light-duty vehicles in China.Methanol can currently be blended up to 3% in gasoline, according to the European standard DIN EN 228, and it is widely regarded as a viable future fuel for marine applications (Schorn et al. 2021).
Dimethyl ether (DME) is one of the simplest ethers, consisting of two methyl group attached to an oxygen atom.It is not a toxic or carcinogenic compound and has a boiling point of 25 °C, however, it is liquid at room temperature under relatively low pressure (≈0.5 MPa) (Catizzone et al. 2017).DME has been noticed as useful and pure fuels for transportation, domestic applications, and power production.With its high cetane number, it can be used as an alternative to diesel fuel (Shikada et al. 2007;Obora and Ishii 2014).What gives DME some advantages that it is safe in storage i.e., it does not form explosive peroxides.Since it does not contain a C-C bond and consists of 35% oxygen, its combustion products are carbon monoxide and unburned hydrocarbon emissions with no emission of toxic gases such as NO x , which are lower than that of natural gas (Azizi et al. 2014).In addition, DME is highly recommended as an environmentally friendly and green refrigerant as it has zero ozone depletion potential and lower global warming potential compared to conventional chlorofluorocarbons (CFCs, Freon) (Lei et al. 2011).Various precursors, such as natural gas, crude oil, leftover oil, coal, and waste products, can be used to produce DME (Rahimpour et al. 2012).There are two popular methods for producing DME; the first method is called the indirect method and consists of two steps with two reactors, where the syngas (CO, H 2 ) are converted to methanol first, and then methanol is converted to DME as shown in reaction ( 11) and ( 13) (Azizi et al. 2014).The direct method by which DME can be produced directly from syngas in a single reactor as presented in reaction ( 14) (Ng et al. 1999).
One of the most promising modern fuel sources for lowtemperature proton exchange membrane fuel cells is formic acid (HCOOH).It is slightly toxic liquid (Robles 2005) and easy to store, providing an additional economic incentive to develop commercially viable direct formic acid fuel cell (Rojas-Andrade and Chen 2018).Formic acid has a wide range of commercial applications, including industry, grass silage, finishing, leather tanning, textile dyeing, natural rubber, food additives, drilling fluids, and a variety of chemical processes (Kawanami et al. 2017).As presented in Fig. 8, different chemicals can be used for formic acid preparation.Carbon monoxide can be used to produce formic acid and this considers the commercial BASF production route.The process contains two reactions.Using a strongly basic sodium methoxide catalyst, CO reacts with methanol to produce methyl formate in the first step then excess water is used to hydrolyze the methyl formate to formic acid.Direct hydrogenation of carbon dioxide in dimethyl sulfoxide (DMSO) with a homogeneous ruthenium catalyst yields 1.9 M formic acid (Moret et al. 2014).

Hydrocarbons
These are organic compounds that contain both hydrogen and carbon.Most hydrocarbons such as gasoline are byproducts of crude oil, and are therefore called petroleum distillates.Many products, including cosmetics, household chemicals, pesticides, fuels, and essential oils, use (13) hydrocarbons as solvents and thinners.There are two types of hydrocarbons: aliphatic (straight chain structures like propane) and aromatic (cyclic structures, such as toluene) (Cubo 2010).In addition to the importance of hydrocarbons as fuels and solvents, one of the most important industrial applications is the production of many polymers and plastics from hydrocarbon monomers such as ethylene and propylene.So, the production of hydrocarbons from the conversion of carbon dioxide represents an industrial leap and a technological transformation.This is because the decline in crude oil production will threaten some industries, especially petrochemical industries, which depend mainly on crude oil refining products.
The simplest hydrocarbon molecule i.e., methane, is a gas with no color or smell, and widely distributed in nature.It makes up the majority of natural gas, which is a mixture of approximately 75% CH 4 , 15% ethane (C 2 H 6 ), and 5% other hydrocarbons such propane (C 3 H 8 ) and butane (C 4 H 10 ) (Pegov 2008).Despite the minor presence of atmospheric methane, it is crucial for regulating the chemistry of the atmosphere by regulating the levels of ozone in the troposphere and acting as a significant sink for hydroxyl radicals in the stratosphere (Scranton and de Angelis 2001).Various supported catalysts have received a lot of attention in CO 2 methanation over the last three decades such as catalysts based on Ru, Rh, Ni, Fe, Co, or Pd.The active metals have been supported by carriers or supports such as SiO 2 , Al 2 O 3 , ZrO 2 , La 2 O 3 , CeO 2 , TiO 2 , MgO, zeolites and carbon compounds (Janke et al. 2014a;Borgschulte et al. 2013;Chang et al. 2001;Song et al. 2010).R. Farrauto and co-workers studied the catalytic activity of 10% Ru/γ-Al 2 O 3 //monolith on the conversion of CO 2 to methane (Janke et al. 2014a).The reaction mixture was 4 vol.%CO 2 and 16 vol.%H 2 .At 217 °C and GHSV = 4720 h −1 , the Ru/γ-Al 2 O 3 catalyst was subjected to eight start-up/shut-down cycles with varying reaction times ranging from 1 to 24 h.At low hydrogenation temperatures, excellent selectivity to methane and CO 2 conversions was achieved under low space-velocity conditions.As a result, the authors proposed that Ru/γ-Al 2 O 3 be used for wash-coated on a heat exchanger to remove the heat liberated.Another research conducted by Sastre and his researchers (Sastre et al. 2014) indicates the possibility of obtaining a conversion rate of over 90% using Nickel supported on silica − alumina and a CH 4 selectivity above 95% using sunlight derived from visible light photo response according to the following reaction.
Gasoline is a middle distillate fraction of crude oil that has been blended with a number of other hydrocarbon and polymer compounds originating from relatively light fractions (C4-C12).Originally, gasoline was a byproduct of the refinery process used to make kerosene from petroleum.It quickly became the preferred automobile fuel due to its high combustion energy (Stauffer et al. 2008;Kaltschmitt and Deutschmann 2012;Lois et al. 2003;Bergendahl 2007).Because of the great importance of gasoline, the process of converting CO 2 into gasoline attracted the attention of scientists, so the researchers focused their research on finding catalysts capable of carrying out this process.Jian Wei (Wei et al. 2017) showed that by using Na-Fe 3 O 4 /HZSM-5 catalyst and a ratio of 1 of feed gas H 2 /CO 2 , high selectivity for gasoline (C5-C11) of 78% and for CH 4 of no more than 4% with CO 2 conversion of 22% were achieved.This could be explained by the catalyst's multifunctionality, which includes three types of active sites with complementary and compatible properties.During CO 2 hydrogenation, CO 2 is first reduced to CO by H 2 via RWGS on Fe 3 O 4 sites, then CO is hydrogenated to α-olefins via FTS on Fe 5 C 2 sites.The olefin intermediates produced by the iron-based catalyst diffuse to the zeolite acid sites, where they undergo acidcatalyzed reactions (oligomerization, isomerization, and aromatization), resulting in the formation of isoparaffins and aromatics that diffuse from the zeolite pores.Another research team has investigated the capability of a multifunctional catalyst made of a mixture of zinc-modified zeolite beta and a methanol producer (InCo) to transform CO 2 into gasoline.A combination of isoparaffinic hydrocarbons was created with a CO 2 conversion of more than 15% and up to 85% iso paraffin selectivity among hydrocarbons (Dokania et al. 2021).
The challenge in converting carbon dioxide to fuel is that it is energy intensive.If we rely on renewable sources to generate energy, we will face two issues: energy storage and fuel transportation.Wind and solar power, for example, require significant energy storage to ensure that electricity can be produced when there is no wind or sun.The most significant advantages of fossil fuels appear here, i.e., their high energy density and ease of storage and transportation (Smit et al. 2014).Another obstruction is the high cost of carbon capture and storage technologies.As well, this technology may not play a sufficient role in rapidly reducing global emissions needed to limit warming to 1.5 °C above pre-industrial levels.To overcome this problem, the development of CCS must be directed in partnership with some sectors such as the petroleum industry as well as iron and cement companies to drive innovation and reduce costs of carbon dioxide capture and storage (Irlam 2017; Hastings and Smith 2020).

Conclusion
There is no doubt that CO 2 gas represents a big problem if it exceeds its permissible normal levels (1.6 tons per capita), but this does not mean that carbon dioxide should be completely eliminated, as CO 2 is the most important greenhouse gas and its presence in the atmosphere trap heat near the earth, which helps the earth to retain some of the energy it gets from the sun, so that it does not lose all the energy to space.The main problem is the increase in its levels, which poses a danger because it leads to global warming and rises the temperature of the planet.Therefore, it is necessary to control the CO 2 gas.For this reason, scientists rushed to recycle CO 2 for two purposes: the first goal is to reduce CO 2 emissions, allowing it to be preserved within safe limits, and the second goal is to work on using it in important industrial applications like fuel as an alternative to fossil fuels, as well as producing some commercially important products such as methanol, diethyl ether and other solvents.For utilizing CO 2 , systems for both capturing and storing CO 2 have been developed, then comes the final stage, which is converting it into valuable products and this depends on the catalysts, which are the process' principal driving force.Given the importance of this topic, future prospects should include more studies to give answers to some inquires such as which of the methods that have been mentioned are most appropriate for separating carbon dioxide, if we have to maintain carbon dioxide at normal levels, how will we maintain the balance between the quantities emitted and consumed, and can we really completely abandon fossil fuels, the main cause of carbon dioxide emissions, and move toward safer alternatives.The world is not far from the negative effects of climate change.Therefore, 198 countries meet annually through the "Conference of the Parties to the United Nations Framework Convention on Climate Change" (COP) to try to find ways to preserve the environment from climate change and to help developing countries combat the effects of climate change.It can take decades to bring new energy technologies to market, but due to the necessity of achieving net-zero emissions globally by 2050, progress must be made considerably more quickly.The government's role includes increasing funding for research and development to achieve zero emissions, providing networks for knowledge exchange, and protecting intellectual property, as well as assisting companies to innovate and invest in infrastructure, and increasing people's awareness of clean energy has become a necessity in our current era to maintain safe levels of emissions.Perhaps it is more likely that the near future will remain largely dependent on fossil fuels as the main source of energy supply, but it is certain when fossil fuels run out, the world will turn to alternatives.There is a need to produce alternative fuels, and one of the important options available is CO 2 conversion.

Fig. 1 a
Fig. 1 a The percentages of CO 2 emissions from different sectors (Ritchie et al. 2020), b The percentage of greenhouse gas (GHG) components (Hockstad and Hanel 2018) and c Illustration of global

Fig. 8
Fig. 8 Formic acid production by different ways

Table 2
Comparison of various Fischer-Tropsch reactors

Table 3
Comparison of the catalytic activity of some different catalysts